Potential and pH Dependencies of Adsorbed Species of 2-, 4-Pyridinethiol and 2-Pyrimidinethiol on Au(111) Electrode

Abstract

Potential and pH dependencies of surface structures of Au(111) electrodes modified with 2- and 4-pyridinethiol (2-PySH and 4-PySH, respectively) and 2-pyrimidinethiol (2-PymSH) were examined by surface-enhanced IR absorption spectroscopy (SEIRAS) and STM. At positive potentials, the pKa values of the adsorbed pyridinethiol and pyrimidine species decreased, and even in an acid solution the pyridine (or pyrimidine) N atom became less protonated. Therefore, STM images of self-assembled monolayers of these thiols observed at properly positive potentials (e.g., > ca. 0.3 V vs. Ag/AgCl for 2-PySH and 2-PymSH in a 0.1 M HClO₄ solution, and >0.7 V vs. Ag/AgCl for 4-PySH in a 0.01 M (M = mol dm⁻³) HClO₄ solution) can be used for evaluating the structures in a neutral solution.